1. Field of the Invention
The present invention relates to a base station apparatus, a communication system and a synchronization method.
2. Description of the Related Art
In a related art communication system (for example, PBX (private branch exchange) wireless system), a communication terminal can move while sequentially switches a base station apparatus (hereinafter referred to merely as “base station”) to be communicated to another among a plurality of base stations. This switching of the base station is called “handover”. In order that the communication terminal preferably conducts the handover, the plurality of base stations needs to mutually synchronize their time information with each other.
As a method of synchronizing the plurality of base stations with each other, the following method has been known. A time information server transmits the time information to the plurality of base stations through a LAN (local area network). Then, each of the base stations adjusts a clock generator installed within the own base station based on a reception time point of the time information and the time information (for example, refer to JP-T-2003-509973).
Likewise, the following method has been also known. A mobile station selectively connects to one or more base stations, and communicates with the connected base stations at given timing. Among the base stations, a base station that generates a transmission timing and a reception timing is set to a higher level of a hierarchy as a reference station. The other base stations other than the reference signal select a base station higher hierarchy, and set the transmission timing and the reception timing. The base stations correct a deviation of the transmission timing and the reception timing from the base station of a synchronization partner at given time intervals (for example, refer to JP-A-2002-165269).
Likewise, the following method has been known. The synchronization server transmits a synchronization message including information on clocks periodically generated to the respective base stations as an IP packet. Each base station receives the synchronization message from the synchronization server. Each base station calculates a clock correction value based on a time at which the synchronization message is received, and the information on the clock notified by the synchronization message, and corrects the clock at each base station based on the clock correction value (for example, refer to JP-A-2004-186877).